FIG. 18 illustrates an injection molding die 102 having a mechanism for rotating a gear piece (molded article piece) 100 during molding of a helical gear or the like and a cooling mechanism 101 for cooling the gear piece 100. The conventional injection molding die 102 illustrated in FIG. 18 has the gear piece 100 in which a cavity 103 for shaping a molded article such as a helical gear is formed and a fixed piece 105 fitted into a fixed piece housing groove 104 (so as to relatively rotatable) formed in the part of the gear piece 100 surrounding the cavity 103. In the conventional injection molding die 102, a cooling water path 106 is formed in the fixed piece 105 and water flowing through the cooling water path 106 of the fixed piece 105 cools the gear piece 100.
In the conventional injection molding die 102 having such a structure, since the gear piece 100 can rotate about the fixed piece 105 and the gear piece 100 is cooled (indirectly cooled via the fixed piece 105) by water flowing through the cooling water path 106 in the fixed piece 105, defective molding caused by the difference in local shrinkage of the helical gear (molded article) is reduced and the cycle time required for injection molding of the helical gear is reduced (see PTL 1).